Ecological Population Growth Rate Model

I am wondering what is wrong with my code. Its simulates an ecological growth rate model. It keeps telling me I have an unexpected symbol and I am not sure where. Here is the code. All the varibles were defined in another line of code so thats not the issue. Some help would be awesome. Thanks!

for (i in 1:length(samples)) { sub_data = subset(lemna, as.character(lemna$Sample.ID) == samples[i]) dN = c(dN, sub_data$N[2:nrow(sub_data)] - sub_data$N[1:(nrow(sub_data)-1)], NA) dt = c(dt, sub_data$t[2:nrow(sub_data)] - sub_data$t[1:(nrow(sub_data)-1)], NA) }

If I invent data that matches what your code needs, it runs for me without any changes.
There is probably an easier way to do what you want but I am not sure what your goal is. In particular, I am not sure why you add an NA at the end of each sub vector.

lemna <- data.frame(Sample.ID = c(1,1,1,2,2,2,2,3,3,3),
                    N = c(2,5,8, 3,9,13,15, 1,2,3),
                    t = c(10, 14, 18, 12, 14, 16, 17, 23, 29, 36))
samples <- c("1", "2","3")
dN <- c()
dt <-  c()
for (i in 1:length(samples)) { 
  sub_data = subset(lemna, as.character(lemna$Sample.ID) == samples[i]) 
  dN = c(dN, sub_data$N[2:nrow(sub_data)] - sub_data$N[1:(nrow(sub_data)-1)], NA) 
  dt = c(dt, sub_data$t[2:nrow(sub_data)] - sub_data$t[1:(nrow(sub_data)-1)], NA) 
}
dN
#>  [1]  3  3 NA  6  4  2 NA  1  1 NA
dt
#>  [1]  4  4 NA  2  2  1 NA  6  7 NA

Created on 2020-10-16 by the reprex package (v0.3.0)